1. Field of the invention
The invention relates to a method of making a complex ceramic core for use in a metal casting process of the type known as the lost wax process, the core having a solid part of ceramic material and at least one recess which extends into the interior of the core, and the core being intended for use in the manufacture of a hollow component comprising internal cavities and partitions, especially a blade for a turboshaft engine.
2. Description of the prior art
In the lost wax casting process use is made of a core of ceramic material which is held in the mold when the metal is cast, the outer surface of the core defining the inner surface of an internal cavity of the finished product obtained in this way.
In some cases, the core used must have recessed portions so as to form partitions in the finished product. Such is the case with cores used in the manufacture of hollow blades for turboshaft engines, these hollow blades having, in their internal cavity, partitions for defining channels for the flow of cooling fluids, and possibly fins for the cooling of the outer walls of the blades.
To make these cores it has already been proposed to make a solid body formed by the core in which the recessed portions are filled by a degradable material, and then to eliminate the degradable composition filling the recessed portions. GB Patent 2 090 181 teaches a method of manufacturing a hollow turboshaft engine blade having an inner partition, in which a partition shape is made from a degradable material by a first injection of degradable material in an appropriate mold, the partition shape of degradable material is molded around by injecting a ceramic based composition in a second mold, and the degradable material is subsequently disposed of by a removal process dependent upon the degradable material used.
However, the method described in this British patent has a few drawbacks and limitations on its use. Firstly, the partition which is made from the degradable material is a thin wall, and this fineness may cause problems when removing the partition from the first mold. Furthermore, the partition may become deformed or may break when injecting the ceramic composition in the second mold as a consequence of the pressures which are exerted on the two faces of the thin wall of degradable material and which do not necessarily balance during the second molding phase. This may result in internal malformation of the casting core, and hence a malformation of the blade made using this core. Finally, in the first injection, it is difficult to produce a complex body comprising a plurality of interconnected thin walls of degradable material, because of undercuts which make it impossible to remove this complex body of degradable material from the mold. This is the reason why turboshaft engine blades which have a complex inner network of partitions are not wholly made by the lost wax casting process. The partitions are made by brazing jackets inside the hollow blade, which involves labor costs, possible changes to the metal and difficulties with ensuring the sealing of the walls.